I found a good paper on St. Thomas's concept of God's knowledge: "God's Knowledge of Future Contingent Singulars: A Reply," by Theodore Kondoleon, in the January 1992 Thomist. (It's available on-line to subscribers; give yourself an early Christmas gift!)

The paper itself is a merciless fisking of an article that appeared in The Thomist a couple of years earlier, but Kondoleon makes a distinction of general value:

...while it is true that God cannot know what is future and contingent as something future and contingent, it is ... because such a knowledge is simply impossible.... not even God can know what is future and contingent as something future and contingent.

Here, I think is the corresponding principle Father Dowd was looking for between God's omnipotence and His omniscience. Just as "God can't make 2+2=5" doesn't mean He is not omnipotent, so "God can't know future contingent being as future contingent being" doesn't mean He is not omniscient.

At the same time, that it is impossible to know what is future and contingent as something future and contingent doesn't mean what is future and contingent can't be known in some other way. Kondoleon gives what I think is a good explanation of the traditional understanding of the way God knows in Himself what for us is future and contingent:

Indeed, as Aquinas himself says in the Summa Theologiae, "So we say that God sees Himself in Himself, because He sees Himself through His essence; and He sees other things not in themselves, but in Himself; inasmuch as His essence contains the similitude of things other than Himself." And elsewhere, in an article in the De veritate addressing the question of God's knowledge of future contingent singulars, he has this reply to one of the objections: "It is true that God knows nothing outside Himself if the word outside refers to that by which He knows. However, God does know something outside Himself if this refers to what He knows." In other words, by knowing their likenesses within Him, their efficient and exemplary cause, God knows the acts of existence which creatures have outside Him.

The paper also clears up, for me at least, how St. Thomas's teaching avoids fatalism:

...Aquinas would insist that, in the case of free agents, God's causality does not "squeeze out" contingency from their free choice acts since God causes these agents to determine for themselves their objects of choice. Acting on the level of being, what the divine causality does cause, as its proper effect, is the actual existence of the free choice act. In causing (as First Cause) the choice act to be actual, He moves the free agent to its act in a manner consonant with the nature of the agent, viz., freely.

I'd accepted before the idea that God can cause an act to be freely chosen, but not really understood how:

In the case of a free choice act, it is the free agent which determines itself to one particular good (real or apparent) as opposed to another, and God moves it to this determination in accordance with His eternal decree to give existence to this act.

The idea, as I now understand it, is that God is the cause of a person's freely chosen act in a way "parallel," so to speak, with the way the person is the cause. It's not that God causes me to freely choose choice A, like a cue ball knocks the nine ball into the two ball to sink the two. Rather, God causes the act of my being free to choose to exist, then causes the act of my freely choosing A to exist, then causes the acts consequent to my freely choosing A to exist. It's a chain of causality distinct from the natural one we (and, I think, Aristotle) normally think of when thinking of cause and effect.

The paper concludes:

Admittedly, there is an apparent contradiction in saying that God moves the free agent to its act of choice and yet it is a free act on the agent's part. However, the divine concurrence does not determine the agent to choose this good as opposed to that, as though the finite agent were not itself responsible for its choice; rather, in moving the agent to its choice act, it confers the actuality of being upon it in accordance with the free determination of the secondary cause. Some claim that there is mystery here rather than logic, but this ought not to be surprising since it involves God's co-causality of His creatures' actions, including those of His free creatures. Truly without Him we can do nothing (not even sin).

"Some claim there is mystery here, but this ought not to be surprising." My sentiments exactly.